So, funny story...
My sister and I decided to 'trade up' our DS systems (for DSi's) in one of those hokey deals that Gamestop puts on. Well, we showed up with our consoles in tow and discussed our intentions with the employee. Unfortunately, it was at this time we realized that my sister's DS system was completely bare, no serial number or evidence of any kind. The discovery was shocking, mostly because of its last minute nature, but also a blow to our pride knowing the system had unknown origins.

I have come across fake consoles before, even ones that look authentic and play the games correctly. Fake DS Lites were common enough a few years ago to make it onto the news.

Apart from the missing sticker, the fact that the cartridge looks like it's sticking out of the system slightly could be a warning sign. It should be flush with the bottom of the console when fully inserted. If it won't go in properly then the console is probably fake.

EDIT: The charger would be another good way of checking, assuming it's the original one that came with the console. I believe it should have the Nintendo logo actually moulded in the plastic on it somewhere and have the code AGS-002. Most 3rd party or fake chargers wouldn't bother.